The present invention relates to a ball bearing which is built in a hard disk drive device (HDD), a video tape recorder (VTR), a digital audio tape recorder (DAT) or the like, to support the spindle rotating at high speed.
For instance, in a hard disk drive device built in a computer, a spindle motor as shown in FIG. 1 is provided to rotate the hard disk driving shaft 1 at high speed. A pair of ball bearings 3 and 3 are provided between the cylindrical surface of the shaft 1 and the inner surface of the housing 2, so as to rotatably support the shaft 1 inside the housing 2. Each of the ball bearings 3 and 3 provides: an inner race 5 of steel having an inner raceway 4 in the outer peripheral surface; an outer race 7 having an outer raceway 6 in the inner peripheral surface; and a plurality of steel balls 8 which are rollingly provided between the inner raceway 4 and the outer raceway 6. The steel balls 8 are pre-loaded so that they may not be fluctuated during the rotation of the shaft 1.
In addition to the above-described spindle motor shown in FIG. 1, spindle motors as shown in FIGS. 2 and 3 have been employed for hard disk drive devices (HDD). In the spindle motor shown in FIG. 2, an outer race 7ahaving a plurality of outer raceways 6 and 6 in the inner cylindrical surface is integral with a hub, and inner raceways 4 and 4 are formed in the cylindrical surface of the shaft 1 and in the outer cylindrical surface of an inner race 5, respectively. Further, in another embodiment of the spindle motor shown in FIG. 2, a plurality of inner races 5 having inner raceways 4 in the outer peripheral surfaces may be provided instead of the inner raceway 4 formed in the cylindrical surface of the shaft 1. According to this embodiment, the spindle motor provides the inner raceways 4 formed in the inner races 5 separated from the shaft 1. Moreover, in the spindle motor shown in FIG. 3, a plurality of inner raceways 4 and 4 are formed in the cylindrical surface of the shaft 1, and a plurality of outer raceways 6 and 6 are formed in the outer peripheral surface of the outer race 7b.
In any one of the above-described spindle motors, the members having the inner raceway 4 and the outer raceway 6; that is, the inner race 5 and the outer race 7 in the case of FIG. 1, the outer race 7a and the shaft 1 and the inner race 5 in the case of FIG. 2, and the shaft 1 and the outer race 7b in the case of FIG. 3 are heretofore made by using high carbon chromium bearing steel class 2, SUJ2 (JIS G 4805). The bearing steel is subjected to so-called "standard heat treatment". That is, after being hardened at temperatures ranged from 820.degree. C. to 860.degree. C., it is tempered at temperatures of from 160.degree. C. to 200.degree. C. In the members thus formed, the raceways are ranged from HRC 58 to HRC 64 in Rockwell hardness, and from 8 to 14% by volume in the amount of retained austenite (.gamma..sub.R).
In the case of a ball bearing used in a part which must be high in corrosion-resistance, its raceway is sometime made by using stainless steel. Examples of the stainless steel for such a raceway are SUS440C (JIS G 4303) and 13 Cr martensite stainless steel. The stainless steel is hardened at a temperature of about 1050.degree. C., and is then subjected to subzero treatment, and tempered at temperatures ranged from 150.degree. C. to 200.degree. C. In the ball bearing thus formed, the raceways are ranged from HRC 57 to HRC 62 in hardness, and from 8 to 12% by volume in the amount of retained austenite (.gamma..sub.R).
Heretofore, the permanent deformation (or plastic. deformation) of the raceway surfaces (the inner and outer raceways 4 and 6) of the bearing rings of a ball bearing of this type is defined in terms of "basic static load rating C.sub.o ". When, with the basic static load rating C.sub.o, the maximum contact surface pressure between the raceway surfaces and the rolling surfaces of the balls 8 exceeds 4000 MPa, the raceway surfaces are adversely affected; that is, they are permanently deformed.
On the other hand, a high precision small ball bearing used in a hard disk drive device (HDD) or a video tape recorder (VTR); that is, a ball bearing which is of at least JIS class 5 in dimensional accuracy or rotational accuracy and of low torque, must be considerably high in sound and noise characteristics. In the high precision small ball bearing, extremely small permanent deformation occurs with the raceway surfaces when a load or impact load is applied thereto which is much smaller than the load (the maximum contact surface pressure of 4000 MPa) specified according to the aforementioned basic static load rating C.sub.o. The permanent deformation deteriorates the acoustic characteristic (increasing noise).
Employed as the high precision small ball bearing as described above is a ball bearing 3 in which, in general, the diameter D.sub.w (FIG. 1) of the balls 8 set between the inner race and the outer race is 3 mm or less, and the pitch circle diameter D.sub.pw (FIG. 1) along which the balls are arranged is 11 mm or less. On the other hand, recently hard disk drive devices (HDD) or video tape recorders (VTR) have been miniaturized, and accordingly the ball bearings used therein have been also miniaturized. Hence, the above-described deterioration in acoustic characteristic caused by the small impact load cannot be neglected.